Ill PORIFERA 



123 



Throughout the group we find the two main layers of cells, gastral and 

 dermal, and everywhere we find the various types of cells which occur 

 in the Ascon. There are characteristic differences in the shape of the 

 spicules and their chemical composition as will appear later. 



A number of different sponges, mostly inhabitants of fresh water, 

 although some of them are marine, possess an interesting adaptive 

 arrangement by which they are enabled to tide over periods of unfavour- 

 able conditions, such as winter in a cold climate, or the dry season in a 

 warm one. This consists in the development of what are known as 

 gemmules and the process is well seen in our common fresh-water sponges. 

 As the period of unfavourable conditions comes on, amoebocytes which 

 have stored up in their cytoplasm large quantities of yolky reserve 





■a^ bI w 



C.c. 



Fig. 57- 



Diagrammatic transverse sections of the Ascon (A) and Sycon (B, C) types of sponge. Tiie layer 

 of clioanocytes is represented by the heavy blacli line, the dermal epithelium by a thin line, and the 

 jelly by fine dots, c.c, Central cavity ; g.c, gastral cavity ; i.c, inhalent canal ; r.c, radiating chamber. 



food-material congregate together in more or less spherical clumps, 

 usually rather less than half a millimetre in diameter and conspicuous 

 to the naked eye from the yellowish colour due to the yolk. Round these 

 spherical masses which are the gemmules there collect other cells of a 

 glandular nature, and apparently by the activity of these the gemmule 

 becomes surrounded by a tough capsule. External to this spicules are 

 collected together to form a further protective envelope. As conditions 

 become more and more unfavourable the ordinary cells of the sponge 

 die away and eventually there is left simply the sponge skeleton with 

 the gemmules scattered through its meshes. This condition persists 

 until conditions again become favourable when the gemmules hatch out, 

 their cells become distributed through the substance of the sponge, they 

 multiply actively and, becoming differentiated in various directions, 



